Electro-hydraulic parallel motion means for drill boom supported rock drilling apparatus



D 2. 1969 B. A. WESTERLUND 3,

ELECTRO-HYDRAULIC PARALLEL MOTION MEANS FOR DRILL BOOM SUPPORTED ROCK DRILLING APPARATUS Filed Nov. 13, 1967 2 Sheets-Sheet 1 Fig.1

INVENTOR. Benqt ARvid WcsicRl and 3pm MM \m.

Dec. 2. 1969 s. A. WESTERLUND 3,481,409

ELECTRO-HYDRAULIC PARALLEL MOTION MEANS FOR DRILL BOOM SUPPORTED ROCK DRILLING APPARATUS Filed NOV. 13, 1967 2 Sheets-Sheet 2 'INVENTOR. Benqt Aavid Westealund aum;

United States Patent U.S. Cl. 17343 13 Claims ABSTRACT OF THE DISCLOSURE In a drill boom for the positioning of a feed shell by actuation of hydraulic rams or actuators, the opposed parallel pivot axles of the jib boomare each associated with angular position sensing means included in an angularly positioning servo system adapted to control automatically the actuation of one of the rams in response to actuation of the other in such manner that the feed shell is translated in parallel relation to itself from drilling position to drilling position vertically or, in case of need, horizontally, as well.

This invention relates to drill booms for angular positioning with respect to a boom support of a feed shell provided with a rock drilling apparatus thereon. More particularly the invention relates to a drill boom which over pivot means perpendicular to one another and preferably disposed horizontally and vertically is pivoted to the boom support as well as to the feed shell and is provided with hydraulic actuating means or rams mounted at the pivot means for adjustment of the angle of elevation and angle of lateral swing on the one hand between the boom support and the drill boom and on the other between the drill boom and the feed shell.

Normally in connection with such drill booms the hydraulic elevation and side swing angle rams of the boom support and the feed shell during adjustment of the feed shell are controlled by separate throttle valves. This asks for experience and is time consuming particularly if it is desired to displace and adjust the feed shell in parallel relation or at a certain constant inclination from hole to hole. It is therefore an object of the invention to make possible servo controlled angular adjustment'of the feed shell in order to control automatically by actuation of a single elevation adjustment throttle valve the setting of the feed shell to the desired final elevation angle position, for example to a position translated vertically in parallel relation to the previous position. It is similarly a further object of the invention to make possible, in case of need, by actuation of single side swing angle adjustment throttle valve that the feed shell also can be set automatically to the desired final side swing angle position, for example to a position translated side-wise in parallel relation to the previous position. Furthermore it shall be possible in connection with the invention to introduce and to maintainin the various drilling positions of the feed shell a difference in angle between the elevation angles and, in case of need, also between the side swing angles at the opposite ends of the drill boom, which differential angle can be selected at will as to its size and direction.

For these and other purposes there is according to the and shell at said parallel pivot means for adjustment of the angles at said pivot means on the one hand between the boom support and the jib boom and on the other between the jib boom and the feed shell, and an angularly positioning servo control system associated with said parallel pivot means for sensing the angular setting at said pivot means and for controlling automatically the actuation of one of said actuating means in response to actuation of the other in such manner that the difference in angle at the parallel pivot means remains substantially unchanged regardless of the angle of adjustment chosen at the pivot means of said other actuating means.

The above and other objects of the invention will become obvious from the following description and from the accompanying drawings in which a preferred embodiment of the invention is illustrated by way of example. It should be understood that this embodiment is only illustrative of the invention and that various modifications thereof may be made within the scope of the claims.

In the drawings FIG. 1 shows partly a side view of a drill boom according to the invention, partly and diagrammatically an electro-hydraulic circuitry for the servo control of the drill boom in respect of the elevation angle adjustment of the feed shell. FIG. 2 shows partly a top plan view of the drill boom in FIG. 1, partly the electric part of an electro-hydraulic circuitry for the servo control of the drill boom in respect of the side swing angle adjustment of the feed shell. FIG. 3 is an enlarged section taken substantially on the line 33 in FIG. 1. FIG. 4 is a side view on the line 44 in FIG. 3.

The drill boom 10 in FIGS. 1 and 2 includes a jib boom 11 which at its ends is pivoted to intermediate pieces 14, 15 over horizontal pivot means or axles 12, 13. The intermediate piece 14 is by means of a vertical pivot means or axle 16 pivoted to and forms part of a boom support 17. The opposite intermediate piece 15 is by means of a vertical pivot means or axle 18 pivoted to a bracket 19 providing a guide for a conventional feed shell 20 slidable longitudinally thereon and adjustable by the aid of a hydraulic feed displacing ram 21. On the feed shell 20 is slidably guided a rock drill 22 which is fed by power for example by a feed screw driven by a feed motor 23.

Across the horizontal pivot axle 12 of the boom support 17 is pivotally coupled a hydraulic elevation angle ram or actuating means 24 which extends between an intermediate portion of the jib boom 11 and the lower end of the intermediate piece 14. By extension or contraction of the hydraulic ram 24 the elevation angle A12 of the jib boom 11 can be adjusted relative to the boom support 17 about the pivot axle 12. A hydraulic elevation angle ram or actuating means 25 is pivotally coupled across the horizontal pivot axle 13 of the feed shell 20 extending from an intermediate portion of the jib boom 11 to the intermediate piece 15. By extension or contraction of the hydraulic ram 25 the elevation angle A13 of the feed shell 20 can be adjusted relative to the horizontal line through the pivot axle 13.

By means of a manually actuatable throttle valve T24 the hydraulic ram 24 is adjustable by being connected to a hydraulic circuit 27 in which there is maintained pressure by a motor driven pump 26. Similarly, the hydraulic ram 25, via conduits 47 and a manually actuatable throttle valve T25, is adjustable by connection to the hydraulic circuit 27. In the conduits 47 to the hydraulic ram there is inserted a pair of solenoid valves SV1 and SV2 in association with which an electric holding circuit 28 for the solenoids of the valves must be broken at a pair of switches B1 and B2 for enabling the valves SV1, SV2 to take, by way of being spring loaded, a position designated by a in which the throttle valve T25 is connected to the conduits 47 and the hydraulic ram 25.

According to the invention there is inserted an angularly positioning servo control system for keeping the divergence between the elevation angles A12 and A13 automatically constant when one changes the elevation angle A12 by the throttle valve T24 and extension or contraction of the hydraulic ram 24. This is equivalent with a parallel translation of the feed shell 20 vertically from drilling position to drilling position. In the angularly positioning servo control system the horizontal pivot axles 12 and 13 are associated each with an electric angular position sensing means R12 and R13, respectively. For the purpose in question may be utilized for example otentiometers, capacitive angle sensing means, angle sensing means of digital type or inductive angle sensing means such as synchros and resolver synchros. In the illustrated embodiment there are used resolver synchros mounted within the pivot axles 12, 13 and shown in detail in FIGS. 3 and 4 in connection with the pivot axle 12.

The resolver synchro R12 is enclosed in a housing which is mounted in pivot axle 12 made hollow and fixed to the latter by screws 31. The screws 31 pass through a cover member 32 which affixes the stator of the resolver synchro 12 to the housing 30. The rotor of the resolver synchro R12 passes with an axle 33 a central journal in the cover member 32 and carries an adjusting lever 34. The adjusting lever 34 coincides with a radial slot 35 on a setting disk 36. The disk 36 is provided with are shaped grooves 37 passed by screws 38 by which the setting disk 36 can be tightened fast in the desired angular position to an inner flange on a hub 39. The hub 39 is affixed by means of screws to the end of the jib boom 11 journalled on the pivot axle 12. When the jib boom 11 turns about the pivot axle 12, the hub 39 and the setting disk 36 thus partake in the rotation and the turning movement is transmitted via the radial slot 35 and the adjusting lever 33 directly to the rotor of the resolver synchro R12. Fine rotor adjustment of the resolver synchro R12 with respect to the stator may be had by screwing ofi an enclosing cover 29 and changing of the tightening position of the screws 38 in the are shaped grooves 37. The windings of the resolver synchro R12 are connected to a cable 40 which is drawn through the pivot axle 12 and the interior of the jib boom 11.

The synchro resolvers R12, R13, mounted in analogous manner in both pivot axles 12 and 13, are for the sake of clearness illustrated in FIG. 1 diagrammatically in a disposition adjacent to the drill boom 10 and the various electric and hydraulic leads and conduits connected to the drill boom are illustrated in developed plan view. In order to provide at the pivot axle 13 servo control of the elevation angle adjustment with respect to the pivot axle 12, the stator windings of the two resolver synchros R12 and R13 may be electrically directly interconnected in per se known manner. For purposes of increasing the possibilities of adjustment and correction in the control of the output angle value it is, however, more advantageous to couple between the resolver synchros R12, R13 electrically a differential resolver synchro DRH which suitably is placed on the control panel, not shown, of the drill boom together with the throttle valves T24, T25 and the switches B1,

B2. The resolver synchro R12, which operates as a transmitter resolver synchro, is connected with its stator windings to the stator windings of the diflerential resolver synchro DRH and the latter with the rotor windings thereof to the stator windings of the resolver synchro R13, which operates as a transformer synchro. The construction of the differential resolver synchro DRH is suitably analogous with the embodiment in FIG. 3 except for the setting disk 36 being graduated and journalled for manual turning adjustment by means of a turning knob 42, FIG. 1, placed on the control panel. One rotor winding of the transmitter resolver synchro R12 is short-circuited while the other is connected to a power unit 43 fed by alternating current. One rotor winding of the transformer resolver synchro R13 is not utilized while the other via leads 44 is connected to an amplifier 45 which is fed by the power unit 43 and by means of leads 46 is connected to the operating motor in an electrohydraulic servo valve HSV intended for elevation angle adjustments. The servo valve HSV is connected to the circuit 27 of the hydraulic pump 26 in parallel with the throttle valve T25. Between the servo valve HSV and the hydraulic ram 25 the solenoid valves SV1 and SV2 are coupled similarly to the arrangement thereof in connection with the throttle valve T25. When the switches B1 and B2 are set to the position depicted in FIG. 1, the electric holding circuit 28 is energized and the valves SV1 and SV2 are moved to the position b against the action of the valve spring load as illustrated in FIG. 1. As a result the throttle valve T28 is coupled off and the hydraulic ram 25 is controlled by the servo valve HSV.

During adjustment one sets, whilst the switches B1 and B2 are open and the solenoid valves SV1 and SV2 are in the position a, the hydraulic rams 24 and 25 in such position by actuation of the throttle valves T24 and T25 that the feed shell becomes parallel with the drilling direction in the drilling pattern in question, for example horizontal and parallel with the direction of the drift. The ditferential resolver synchro DRH is set to zero by means of the knob 42 and the transformer resolver synchro R13 is adjusted by means of the setting disk 36 in such manner with respect to the transmitter resolver synchro R12 that the output signal in the lead 44 will be zero. This means that the angles A12 and A13 are equal and that the difierence therebetween equals zero. Thereupon the switches B1 and B2 are set in such position that the solenoid valves SV1 and SV2 will take position b, FIG. 1.

If the elevation angle A12 of the jib boom 11 now is changed by manual actuation of the throttle valve T24, the orientation of the alternating electromagnetic field generated by the alternating currents through the rotor winding in the transmitter resolver synchro R12 will be changed as a result of the turning of the jib boom 11. The angular turning of this field causes a correspond ing turning of the electromagnetic field induced by the rotor in the stator windings of the transmitter resolver synchro R12, and the turning of the stator field causes in its turn, since full reproduction of the field turning takes place in the differential resolver synchro DRH, being zero adjusted with respect to the output field angle difference, a turning to an equal extent of the electromagnetic field generated by the currents in the stator windings of the transformer resolver synchro R13. The angular orientation of the alternating electromagnetic field of the stator windings R13 will in the new angular position divert in such manner from the starting value that there is induced an error voltage in the windings of the transformer resolver synchro R13 connected to the leads 44 which error voltage corresponds in magnitude to the change in angle. The error voltage is transmitted via the leads 44 to the amplifier 45 fed by the power units 43, in which amplifier 45 the error voltage is amplified and detected as to its phase position with respect to the voltage by which the rotor in the transmitter resolver synchro R12 is fed in order finally to be rectified so that the polarity of the direct current thus created will be dependent upon in which sense the rotor in the transmitter resolver synchro R12 had been turned. The error voltage thus rectified is fed via the leads 46 to the operating motor in the electro-hydraulic servo valve HSV which thus performs a control movement from its neutral position in such direction that the hydraulic ram 25 via the intermediate piece 15 will turn the 'feed shell in the direction necessary for again making the angle A13 equal to the angle A12. One thus receives an automatic parallel keeping means of the feed shell regardless of the elevation angle setting at the hydraulic ram 24 and the pivot axle 12. If it is desired to incline the feed shell 20 for example for purposes of drilling inclined roof holes or bottom holes or for correcting the horizontal position of the feed shell, one actuates the knob 42 of the ditferential resolver synchro DRH whereby an angular divergence is introduced between the electromagnetic stator field in the differential resolver synchro DRH and the field generated by the induced rotor currents of the differential resolver synchro DRH in the stator of the transformer resolver synchro R13, which consequently causes a corresponding angular divergence between the angles A12 and A13. The angular divergence thus adjusted will be maintained unchanged when the hydraulic ram 44 is actuated for. changing the angle A12.

It is obviously possible to maneuver in analogous manner and to control by servo means the side angle adjustment of the drill boom. This is illustrated diagramrnatically in FIG. 2. The intermediate piece 14 of the boom support 17 is via an arm pivotally coupled to a hydraulic side swing angle rarn or actuating means 48- fixed to a pivot point on the boom support 17. By extension or contraction of the hydraulic side swing angle ram 48 is performed a lateral swing of the jib boom 11 and an adjustment of its side swing angle A16. At the outer end of the jib boom 11 there is coupled across the pivot axis 18 a hydraulic side swing angle ram or actuating means 49 between the intermediate piece 15 and the bracket 19. By extension or contraction of the hydraulic side swing angle ram 49 the side swing angle A18 of the feed shell 20 can be adjusted. In the pivot axles 16 and 18 there are, similarly to the embodiment in FIG. 1, mounted resolver synchros R16 and R18 and also in the case of FIG. 2 there is provided a hydraulic system, not shown, analogous with the embodiment in FIG. 1. The resolver synchros R16 and R18 are here, as well, connected electrically to a dilferential resolver synchro DRS provided with a turning knob 50 placed at the control panel of the drill boom.

The rotor Winding of the resolver synchro R18 is via a lead 51 connected to an amplifier 52 which similarly to the embodiment in FIG. 1 controls an electro-hydraulic servo valve HSS intended for side. angle adjustment and forming part of the hydraulic system. The automatic servo .control of the side angle A18 in dependence upon the adjustment of the side angle A16 will be fully analogous with the above described servo control of the elevation angle A13.

In the use of synchros of three phase type instead of two phase resolver synchros for the parallel translation,

there is used an electro-hydraulic wiring diagram which in rinciple is similar to the above described system with resolver synchros. If it is desired on the other hand to use Potentiometers, digital angular position sensing means, or such means of capacitive type, the electric parts ofthe servo control should be adapted. to the type of angular position sensing means chosen in a manner per servo valve can be arranged for operation in on-off control with full deflection from the neutral position when the incoming error signal exceeds a certain threshold value.

I claim:

1. A drill boom comprising a boom support, an elongated feed shell, a feed motor on said feed shell, a rock drill slidably supported on said feed shell and operatively connected to said feed motor for reciprocal movement along said feed shell, a jib boom having parallel pivot axles at the opposed ends thereof connecting said jib boom pivotally to the boom support as well as to the feed shell, hydraulic rams pivotally coupled to said jib boom and respectively to said support and shell across said parallel pivot axles, manually actuatable hydraulic control means connected to each ram for adjustment of the angles at said axles on the one hand between the boom support and the jib boom and on the other between the jib boom and the feed shell, and an angularly positioning self-correcting servo control system including means associated with said parallel pivot axles for sensing the angular setting at said pivot means and for controlling automatically the actuation of one of said rams in response to actuation of the other in such manner that the difference in angle at the pivot axles remains substantially unchanged regardless of the angle of adjustment chosen at the pivot axle across which said other ram extendsi 2. A drill boom according to claim 1 in which said servo control system is electrohydraulic including electrically interconnected synchros connected to said pivot axles for sensing the angular setting thereof, and an electro-hydraulic servo valve connected electrically to one of said synchros for being actuated thereby and for controlling hydraulically the actuation of said one actuating means.

3. A drill boom comprising a boom support, an elongated feed shell, a feed motor on said feed shell, a rock drill slidably supported on said feed shell and operatively connected to said feed motor for reciprocal movement along said feed shell, a jib boom having parallel pivot axles at the opposed ends thereof connecting said jib boom pivotally to the boom support as well as to the feed shell, hydraulic rams pivotally coupled to said jib boom and respectively to said support and shell across said parallel pivot axles for adjustment of the angles at said axles on the one hand between the boom support and the jib boom and on the other between the jib boom and the feed shell, and an angularly positioning servo control system including means associated with said parallel pivot axles for sensing the angular setting at said pivot means and for controlling automatically the actuation of one of said rams in response to actuation of the other in such manner that the difierence in angle at the pivot axles remains substantially unchanged regardless of the angle of adjustment-chosen at the pivot axle across which said other ram extends, said angularly positioning self-correcting servo control system being electrical and including a transmitter synchro associated with the pivot axle across which said other ram extends, a control transformer synchro electrically connected for angular synchronization with said transmitter synchro and associated with the pivot axle across which said one ram extends, an electrohydraulic servo valve including an electric operating motor therefor connected for controlling hydraulically the actuation of said one ram upon electric actuation of said operating motor, and an electric control circuit including an amplifier between said transformer synchro and said operating motor for actuating the latter electrically.

4. A drill boom according to claim 3, in which an angularly adjustable differential transmitter synchro is included in the electric connection between said transmitter and control transformer synchros for selective setting as to size and direction of the difference in angle to be kept substantially unchanged at said parallel pivot axes.

5. A drill boom according to claim 3 in which said transmitter and transformer synchros are respectively resolver transmitter and transformer synchros.

6. A drill boom according to claim 4 in which said dififerential transmitter synchro is a resolver synchro.

7. A drill boom comprising a boom support, an elongated feed shell, a feed motor on said feed shell, a rock drill slidably supported on said feed shell and operatively connected to said feed motor for reciprocal movement along said feed shell, a jib boom having horizontal pivot means at the opposed ends thereof connecting said jib boom vertically pivotally to the boom support as well as to the feed shell, hydraulic actuating means coupled to said jib boom and respectively said support and shell at said pivot means for adjustment of the angle of elevation on the one hand between the boom support and the jib boom and on the other between the jib and boom and the feed shell, and an electro-hydraulic angularly positioning servo control system including a transmitter synchro associated with said pivot means at said boom support and a control transformer synchro associated with said pivot means at said feed shell for sensing the angular setting at said pivot means and for controlling automatically the hydraulic actuating means coupled to said feed shell in response to hydraulic actuation of the hydraulic actuating means coupled to said boom support in such manner that the :diflerence in elevation angle at said pivot means remains substantially unchanged regardless of the elevation angle chosen at the horizontal pivot means of said boom support.

8. A drill boom comprising a boom support, an elongated feed shell, a feed motor on said feed shell, a rock drill slidably supported on said feed shell and operatively connected to said motor for reciprocal movement along said feed shell, a jib boom having horizontal and vertical pivot axles at the opposed ends thereof connecting said jib boom pivotally in respectively vertical and horizontal directions to the boom support as well as to the feed shell, hydraulic rams pivotally coupled to said jib boom and respectively to said support and shell across said pivot axles, manually actuatable hydraulic control means connected to each ram for adjustment of the angle of eleva tion and angle of lateral swing on the one hand between the boom support and the jib boom and on the other between the jib boom and the feed shell, and first and second angularly positioning self-correcting servo control systems with the first of them associated with the horizontal pivot axles and the second with the vertical pivot axles for sensing the angular setting at said pivot axles and for controlling automatically the actuation of respectively the elevation and side swing angle rams at said feed shell in response to actuation of respectively the elevation and side swing angle rams at said boom support in such manner that the difference in respectively elevation and side swing angle at said pivot axles remains substantially un changed regardless of the elevation and side swing angles chosen at the pivot axles of said boom support.

9. A drill boom comprising a boom support, an elongated feed shell, a feed motor on said feed shell, a rock drill slidably supported on said feed shell and operatively connected to said motor for reciprocal movement along said feed shell, a jib boom having horizontal and vertical pivot axles at the opposed ends thereof connecting said jib boom pivotally in respectively vertical and horizontal directions to the boom support as well as to the feed shell, hydraulic rams pivotally coupled to said jib boom and respectively to said support and shell across said pivot axles, for adjustment of the angle of elevation and angle of lateral swing on the one hand between the boom support and the jib boom and on the other between the jib boom and the feed shell, and first and second angularly positioning servo control systems with the first of them associated with the horizontal pivot axles and the second with the vertical pivot axles for sensing the angular setting at said pivot axles and for controlling automatically the actuation of respectively the elevation and side swing angle rams at said feed shell in response to actuation of respectively the elevation and side swing angle rams at said boom support in such manner that the difference in respectively elevation and side swing angle at said pivot axles remains substantially unchanged regardless of the elevation and side swing angles chosen at the pivot axles of said boom support, wherein said first and second servo system each includes electric angular positioning sensing means associated one with each of the parallel pivot axles belonging to the system in question and disposed at the opposed ends of said jib boom, the sensing means in each system associated with the pivot axles at said feed shell being coupled electrically to the sensing means of the system associated with the pivot axle at the boom support for delivering an electric output error signal when the angular difference at said pivot axles is changed due to hydraulic actuation of the hydraulic ram extending across said pivot axle of said boom support, an electro-hydraulic servo valve in said first and second servo system and including an electric operating motor therefor, means connecting said servo valve in each system hydraulically to the hydraulic ram extending across the pivot axle of said feed shell for controlling the hydraulic actuation of said ram upon electric actuation of said operating motor, and an electric connection in each system between said sensing means at the pivot axle of said feed shell and said operating motor and including an amplifier for causing said output error signal to actuate said operating motor.

10. A drill boom according to claim 9 in which the angular position sensing means are resolver synchros,

11. A drill boom according to claim 10 in which an angularly adjustable differential resolver synchro is included in each servo system in the electric connection be-. tween the two resolver synchros thereof for selective setting as to size and direction of the diiference in angle to be kept substantially unchanged at said parallel pivot axles.

12. A drill boom comprising a boom support, an elongated feed shell, a feed motor on said feed shell, a rock drill slidably supported on said feed shell and operatively connected to said feed motor for reciprocal movement along said feed shell, a jib boom having parallel pivot axles at the opposed ends thereof connecting said jib boom pivotally to the boom support as well as to the feed shell, hydraulic rams pivotally coupled to said jib boom and respectively to said support and shell across said parallel pivot axles for adjustment of the angles at said axles on the one hand between the boom support and the jib boom and on the other between the jib boom and the feed shell, and an angularly positioning servo control system including means associated with said parallel pivot axles for sensing the angular setting at said pivot axles, means for synchronizing said sensing means to an angular adjustment corresponding to a predetermined desirable ditference in angle at said pivot axles, and means responsive to angular error during said synchronization for controlling automatically the extension or contraction of one of said rams in response to actuation of the other, said actuation of said one ram being directed such as to eliminate said angular error. i

13. A drill boom comprising a boom support, an elongated feed shell, a feed motor on said feed shell, a rock drill slidably supported on said feed shell and operatively connected to said motor for reciprocal movement along said feed shell, a jib boom having horizontal and vertical pivot axles at the opposed ends thereof connecting said jib boom pivotally in respectively vertical and horizontal directions to the boom support as well as to the feed shell, hydraulic rams pivotally coupled to said jib boom and respectively to said support and shell across said pivot axles for adjustment of the angle of elevation and angle of lateral swing on the one hand between the boom support and the jib boom and on the other between the jib boom and the feed shell, and first and second angularly positioning servo control systems with the first of them including means associated with the horizontal pivot axles and the second including means associated with the vertical pivot axles for sensing the angular setting at said pivot axles in both said servo control systems, each said servo control system further including means for synchronizing said sensing means thereof to an angular adjustment corresponding to a predetermined desirable difference in angle at said pivot axles thereof, and means responsive to angular error arising in said synchronization for controlling automatically extension or contraction of one of said rams coupled across the pivot axles of said system in question in response to actuation of the other of said rams, said actuation of said one ram being directed such as to eliminate said angular error.

References Cited UNITED STATES PATENTS 3,338,316 8/1967 Thompson 173-43 3,374,975 3/1968 Bronder 173-38 X FOREIGN PATENTS 1,498,152 10/1967 France.

ERNEST R. PURSER, Primary Examiner US. Cl. X.R.

P0405" UNITED STATES PATENT OFFICE 56 CERTIFICATE OF CORRECTION Patent No. 3,481J409 Dated D c mb r 1969 Inventm-(s) NES'IERLUND It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

- Column 7 line 16, delete the word "and" as it appears after ",jib'.

SIGNED MI'D SEALED Jun 2 497 m) Attest:

Edwan'i M. Fl m WILLIAM E. 50mm, 1!. Attestmg Offlcer Comissioner of Patenta 

